121–127 of 127 posts
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Who has unlimited land?
I'm in Australia. We have unlimited land and enough sun to power the solar system.
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Sure, you can increase your efficiency, but: There's a hard upper limit. 1 kW/meter^2 (mis-typed as 1W above). And you've got diminishing marginal returns of more efficient cells. The truth is that the land requirements are likely to go down by a factor of 2-4 maximum. Other cost factors (transmission, but especially storage) will dwarf these. Once you've provisioned land for solar, the key costs are in replacing the…
Germans in their Kombikraftwerk study back in 2007, figured out that you need 5x the capacity (compared to the average usage) to cover entire annual (and daily) fluctuations of the grid with renewables, without storage (more exactly, with that little storage they already have - in the form of a few pumped hydro plants). I tend to still trust this study, there are no counter-arguments to speak of.
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Germans in their Kombikraftwerk study back in 2007, figured out that you need 5x the capacity (compared to the average usage) to cover entire annual (and daily) fluctuations of the grid with renewables, without storage (more exactly, with that little storage they already have - in the form of a few pumped hydro plants). I tend to still trust this study, there are no counter-arguments to speak of.
I believe that, assuming the study includes wind and biomass. But 5x capacity is going to be expensive to maintain.
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In practice it's pretty bad; Hydrogen, even compressed, takes up a large amount of volume. A 10 gallon tank stores about 1.5 gge of compressed H2. The energy to compress H2 is again as much as it takes to create H2, and you don't get that energy back when you burn it (combustion perhaps 30% efficient). An electric battery returns a far higher percentage of the power in, and is far simpler and safer. H2's dangers are…
I don't see H2 as a highly efficient storage option, but if the alternative is simply wasting the electricity generated, it's a choice worth considering. Compression or liquification will address some of the storage density issues, combustion is straightforward. If it's being used on-site or nearby then transport and handling are reasonably minor issues -- wider-scale distribution is a thorny problem. In an oil-scarc…
Compression or liquefaction don't address the storage density issues. The horrible densities involved are _after_ you put the Hydrogen through those processes to their fullest possible physical limit. Uncompressed Hydrogen in a 10 gallon tank contains about 0.003 gge (enough energy to move an already-moving car just 500 feet).
Aircraft fuels are being addressed by biofuels including crop-based, algae-based, or char-based kerosene-like compounds. Natural gas-derived (steam reforming) and coal-derived (Fischer-Tropsch or Karrick) Jet A are already possible and much more scalable than oil-derived, just more expensive for now.
Hydrogen could never practically power a performance airplane (rocket is a different story) simply because you need big heavy tanks to safely carry it.
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I don't see H2 as a highly efficient storage option, but if the alternative is simply wasting the electricity generated, it's a choice worth considering. Compression or liquification will address some of the storage density issues, combustion is straightforward. If it's being used on-site or nearby then transport and handling are reasonably minor issues -- wider-scale distribution is a thorny problem. In an oil-scarc…
That's just it- the alternative is never wasting the electricity; you wouldn't invest thousands to millions of dollars in half a system. Well, I wouldn't. Compression or liquefaction don't address the storage density issues. The horrible densities involved are _after_ you put the Hydrogen through those processes to their fullest possible physical limit. Uncompressed Hydrogen in a 10 gallon tank contains about 0.003 g…
Well: don't discard unused what you can profitably exploit. That is: you get more utility from the storage than it costs you to get it. Net energy on storage will always be less than the input, but so long as it's either net positive or you get some highly useful form of energy out (food, liquid fuels, chemical feedstocks), it's worthwhile.
storage density issues I said "some". Compressed / liquified H2 is viable for some uses. Including flight. Remember, the alternative isn't existing fossil-fueled heavier-than-air craft, but airships and other "unconventional" fuels.
Aircraft fuels are being addressed by biofuels
If there were any level of success from these efforts I'd expect them to be touted to the stars. Pretty much every pilot project I've heard of (one big one in the US Midwest over the past year or so) has been exceptionally quiet/muted. The costs are going to be very high, and I'm expecting roughly $1000/bbl, translating likely to $50/gallon fuel. At 40 passenger miles to the gallon, a transcontinental (3000 mile) flight would run you $3750 in fuel charges alone.
Conventional freight rail "moves 1 ton of goods 100 miles on 1 gallon of gasoline". Assuming 180# per person, that's about 1100 miles per person per gallon, or 2.7 gallons for a transcontinental trip ($135 in fuel costs at $50/gallon). I'd expect that passenger rail achieves only a fraction of the efficiency of freight (lower packing densities, more stops, etc.). Turns out it's quite a bit less according to methodology applied to Amtrak. Roughly 55 passenger miles/gallon, a high of 80 pmg during WWII given higher utilization rates: http://www.railway-technical.com/US-fuel-paper.shtml
This could likely be improved by reducing amenity cars (lounge, dining, observation). And it's frankly not much better than a personal automobile with 2 or more occupants.
However: trains offer one significant advantage over aircraft. They can be electrically powered. Which frees them from dependence on (increasingly rare and expensive) liquid fuels. With conventional (<80mph), "higher speed" (<125 mph), or "high speed" rail (150 - 220 mph), power consumption is around 50-95 kWh/passenger km (todo: convert to mpg equivalent), with loading factor (how many seats are filled) being a key determinant. At 350 kph, a non-stop transcontinental trip would be roughly 14 hours (adding in stops and dwell time would increase this, though if kept to a minimum, not by much). Hardly as convenient as the 5-6 hours presently attainable, but you'd have more space and amenities, as well as the option to embark and debark directly in city centers. An overnight service could be feasible: leave at 6pm, arrive at 5am (heading west) or 11 am (heading east). Stagger service a bit -- you could depart at 4pm for east-bound and 8pm for west-bound service to make arrivals more convenient.
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Exactly. So lets talk about a different, important number. Watts per dollar per day. The only one your accountant/investor cares about. Its something like double or triple for space-based solar cells, if you can deploy them efficiently.
a. they have to be in space so your capital outlay is insane b. you have massive efficiency losses in wireless power transmission
As for efficiency losses, I don't know what figures you used but its essentially the same problem to receive photons on the ground from a maser in space as the original task of converting sunlight in the first place. Efficiencies there are in fact very high (re: OP).